1. Technical Field
The invention relates generally to production of optical lenses for use in microlithography systems and other applications requiring transmission of ultraviolet radiation. More particularly, the invention relates to production of bulk fused silica for use in fabricating the optical lenses.
2. Background Art
Fused silica is well known for its use in the production of large telescope mirrors and optical fibers. Fused silica is also used for producing other optical elements such as lenses. Recently, fused silica has found a variety of uses in applications requiring transmission of ultraviolet radiation. One such application is in microlithography systems, which are employed in the production of integrated circuits in the semiconductor industry. These systems use multiple fused silica lenses, called stepper and scanner lenses, to transmit radiation from excimer lasers to photosensitized silicon wafers. It is important that the lenses are made from high-purity fused silica because impurities in the lenses can distort the images projected onto the wafers, as well as change the optical characteristics of the lenses.
Several patents have issued that describe methods for producing fused silica lenses for use with excimer lasers. U.S. Pat. No. 5,616,159 issued to Araujo et al. describes a method of forming an optical member or blank for use with light having a wavelength range shorter than about 300 nm. The method consists essentially of forming a blank from high-purity synthetic silica glass containing hydroxyl (OH) groups in an amount no greater than 10 ppm by weight and doping the formed blank with molecular hydrogen to increase the resistance of the optical member to laser damage. U.S. Pat. No. 5,896,222 issued to Rosplock et al. discloses a method of producing a fused silica glass lens that transmits ultraviolet radiation having a wavelength below 300 nm with controlled optical damage and inhibited red fluorescence during such transmission. The fused silica glass lens has hydrogen content below 1018 molecules/cm3 and is made by thermal conversion of a polymethylsiloxane precursor.
U.S. Pat. No. 5,364,433 issued to Nishimura et al. discloses a method for producing a synthetic quartz glass optical member for an ultraviolet laser. The method consists of forming a quartz glass with OH group content ranging from 10 to 100 ppm, a chlorine content of not more than 200 ppm, a hydrogen molecule content of not more than 1016 molecules/cm3, a uniformity refractive index distribution of not more than 5xc3x9710xe2x88x926 as expressed in terms of xcex94n, and a birefringence of not more than 5 nm/cm. The quartz glass is made by thermal conversion of volatile silicon compounds to fine particles of silica glass, followed by deposition of the silica particles on a heat-resistant substrate to form a rod-like porous ingot of synthetic silica glass. Examples of the volatile compounds include alkoxysilanes or alkylpolyalkoxysilanes, tetramethoxysilane, silane compounds, and volatile inorganic silicon compounds, e.g., silicon tetrachloride.
Generally, processes for producing fused silica comprise providing a feedstock solution, generating vaporous reactants from the feedstock, transporting the vaporous reactants to a reaction site, and converting the vaporous reactants to fine silica particles by thermal decomposition with oxidation and/or flame hydrolysis. Thermal decomposition with oxidation involves passing the vaporous reactants and an oxidant through a zone that is heated to at least the pyrolytic temperature of the vaporous reactants. The heat decomposes the vaporous reactants to silicon and other elements, and the oxidant combines with silicon to produce the silica particles. In flame hydrolysis, the vaporous reactants are introduced into a flame formed by combustion reaction of a hydrogen-containing fuel with oxygen. The combustion reaction results in production of sufficient water to hydrolyze the vaporous reactants or oxygen to oxidize the vaporous reactants. The combustion reaction also results in production of sufficient auxiliary heat to promote the thermal environment necessary to produce the silica particles.
The silica particles can be immediately deposited on a hot bait or crucible to give a non-porous, transparent, bulk glass, commonly called a boule. The boule can be used individually to fabricate optical elements such as lenses or finished and integrated into large optical bodies such as telescope mirrors. Alternatively, the silica particles can be deposited on a heat-resistant substrate to give a porous silica ingot, commonly called silica soot. A porous silica ingot can also be produced using vapor-phase axial deposition technique, outside vapor deposition technique, plasma-activated chemical vapor deposition technique, and a host of other chemical vapor deposition techniques. The porous silica ingot can be subsequently thermally consolidated, molded, and heat-treated to give a non-porous, transparent, glass article.
It has been found that the selection of the feedstock used in the production of fused silica is as important as the design of the equipment used to produce the fused silica. For a long time, the standard feedstock used in the production of fused silica was silicon tetrachloride (SiCl4). SiCl4 was selected because it yielded large amounts of vapors at low temperaturesxe2x80x94SiCl4 has vapor pressures of 200 to 300 mm Hg in a temperature range of 21xc2x0 C. to 31xc2x0C. Using SiCl4 as a precursor in flame hydrolysis, however, has a drawback in that the by-product of the process is hydrochloric acid (HCl), an environmentally unfriendly material that requires considerable care for its disposal. In recent years, several processes for producing fused silica using halide-free feedstock have been proposed. These halide-free production processes were developed in response to increased sensitivity to the environment and stricter government regulations.
U.S. Pat. No. 5,043,002 issued to Dobbins et al. discloses a process for making fused silica using polymethysiloxane, in particular, octamethylcyclotetrasiloxane (OMCTS), which is represented by Si4O4(CH3)8. The process is illustrated in FIG. 1. First, OMCTS 1 is vaporized by heating at 100-150xc2x0 C. and bubbling with an inert carrier gas 2b, e.g., nitrogen. A stream of inert gas 2a, e.g., nitrogen, is brought into contact with the OMCTS 1 vapors to prevent saturation of the vapors. The streams of gases 2a, 2balong with the OMCTS 1 vapors are carried to a distribution mechanism 3, which in turn transports the OMCTS 1 vapors to a reaction site. The reaction site includes a number of burners 4 which fire into a furnace crown 5. A fuel/oxygen mixture 0 is conducted to the burners 4 and exit burner orifices which are separate from those used in the burning of OMCTS. The burning of the fuel and OMCTS results in silica particles. The silica particles are directed downwardly and immediately deposited and consolidated into a non-porous mass on a hot bait 6. The by-products of the process are carbon dioxide and water.
U.S. Pat. No. 5,152,819 issued to Blackwell et al. discloses a process for making fused silica using an organosilicon-R compound as feedstock, where R is an element of the periodic table. The preferred organosilicon-R compound has the following properties: (1) a Sixe2x80x94R bond dissociation energy that is no higher than the dissociation energy of the Sixe2x80x94O bond, (2) a boiling point that is no higher than 350xc2x0 C., and (3) produces, upon pyrolysis and/or hydrolysis, decomposition products besides SiO2 which are considered to be environmentally safe. Three classes of organosilicon-R compounds were proposed, including organosilicon-oxygen compounds having a basic Sixe2x80x94Oxe2x80x94Si structure, organosilicon-nitrogen compounds having a basic Sixe2x80x94Nxe2x80x94Si structure, and siloxasilazanes having a basic Sixe2x80x94Nxe2x80x94Sixe2x80x94Oxe2x80x94Si structure. These compounds have a significant vapor pressure when heated above 150xc2x0 C. A number of organosilicon-R compounds were disclosed, including tris ketenimine, nonamethyltrisilazane, octamethylcycotetrasilazane, and hexamethylcyclotrisiloxazane, a siloxasilazane. Non-porous mass of fused silica is produced using a process similar to the one disclosed by Dobbins et al., except that OMCTS is replaced by an organosilicon-R compound that satisfies the criteria above.
U.S. Pat. No. 4,038,370 issued to Tokimoto et al. discloses a process for making high-purity transparent vitreous silica using a high-purity, silane-type gas. Examples of high-purity, silane-type gases used in the process include silane (SiH4), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), and silicon tetrachloride (SiCl4). The vitreous silica is produced by supplying the silane-type gas, an inert gas, hydrogen gas, and oxygen gas to a burner to carry out flame hydrolysis. The flow rate of the hydrogen gas was maintained at least six times the flow rate of the silane gas, while the flow rate of the oxygen gas was maintained less than 1.5 times a theoretical supply flow rate required for completely oxidizing the silicon in the silane and the hydrogen existing during the flame hydrolysis process. The vitreous silica obtained from the flame hydrolysis process included metastable OH. The vitreous silica was heat treated at a temperature higher than 800xc2x0 C. in an inert atmosphere to reduce the hydroxyl (OH) content of the vitreous silica to discharge the metastable OH and reduce the OH content to a range from 40 to 60 ppm.
U.S. Pat. No. 3,117,838 issued to Sterling et al. discloses a process for making fused silica or quartz by thermal decomposition with oxidation of silane (SiH4). In one of the disclosed embodiments, silane and oxygen are fed into a burner or torch jet and the flame is allowed to impinge on a carbon substrate upon which silica is to be deposited. The flame is formed by spontaneous ignition of silane with oxygen. Silane is combusted to form silica particles, which accumulate on the carbon substrate to form a solid body of transparent silica. It is not necessary to vaporize silane because silane exists as vapor at room and elevated temperatures. Oxygen is supplied to the interior and exterior of the jet of silane to ensure complete combustion of silane. The flow of oxygen and the flow of silane are regulated to ensure an excess of about 25% by volume of oxygen. In other embodiments, silane is reacted with carbon dioxide, nitrous oxide, or water to produce high purity material.
A method for preparing high-purity, bulk fused silica includes supplying silane gas, a gaseous fuel, and oxygen gas to a combustion burner and forming silica particles by passing the silane gas into a flame formed by the combustion reaction of the gaseous fuel with the oxygen gas while maintaining the ratio of the flow rate of the gaseous fuel to the flow rate of the silane gas no less than twelve and the ratio of the flow rate of the gaseous fuel to the flow rate of the oxygen gas no less than two. The method further includes immediately depositing the silica particles onto a hot bait to form a boule.